Accuracy Assessment in Assisted Gps Positioning

ABSTRACT

Reliable and efficient search windows are provided by allowing the adaptation of the code search window to be dependent on inaccuracy measures of relations between a cellular frame time and a satellite reference time. This inaccuracy is calculated in a positioning node ( 21 ) of the cellular communications system ( 1 ), preferably by filtering of measurements received from user equipments. Linear trend Kalman filtering followed by post processing of estimation errors is presently preferred. In order to ensure non-ambiguous interpretation of the received time stamps of received satellite signals ( 55 ) provided by user equipments ( 10 ), a pseudo propagation delay is computed in both the user equipment ( 10 ) and the positioning node ( 21 ) based on GPS acquisition assistance data. The GPS time stamp is then defined referring to the determined pseudo propagation delay. In a preferred embodiment, the pseudo propagation delay is assured to be situated within a pre-determined time interval.

TECHNICAL FIELD

The present invention relates in general to the field of satellite basedpositioning, and in particular satellite based positioning assisted byassistance data from a cellular communication system.

BACKGROUND

Positioning or navigation technology has the purpose of determining ageographic position of an object, equipment or a person carrying theequipment. The position is typically given with respect to a specifiedcoordinate system. Such positioning has become more and more interestingin many fields of applications during recent years. One approach tosolve the positioning is to use signals emitted from satellites todetermine a position. Well-known examples of such systems are the GlobalPositioning System (GPS) and the coming GALILEO system. The position isgiven with respect to a specified coordinate system as atriangulation/trilateration based on a plurality of received satellitesignals.

Assisted GPS (AGPS) is an enhancement of the GPS system, to facilitateintegration of GPS receivers into e.g. mobile terminals of cellularand/or cellular communication systems. The GPS reference receiversattached to a cellular communication system collect assistance datathat, when transmitted to GPS receivers in terminals, enhance theperformance of the GPS terminal receivers. Additional assistance data iscollected from the cellular communication system directly, typically toobtain a rough initial estimate of the position of the terminal togetherwith a corresponding uncertainty of the initial estimate. This positionis often given by a so called cell identity positioning step, i.e. theposition of the terminal is determined with cell granularity.

Fine time assistance means that the GPS receiver is provided with highlyaccurate information or data related to a satellite time reference, e.g.the global GPS time, and satellite positions in space. This, in turn,allows upper and lower bounds of a search window for the code phases ofsignals transmitted from all GPS satellites to be computed for terminalsthat reside anywhere in a region obtained by an initial, relativelyinaccurate positioning step. This follows since the times oftransmission of the signals from the GPS satellites are synchronizedwith extreme precision, and since the orbits of these satellites can becalculated in the cellular communication system using other types ofassistance data obtained from e.g. GPS reference receivers.

There are two main sources of errors present in this process. The firstis caused by the fact that the initial position of the user equipment isnormally not known with better accuracy than the size of the cell towhich it is connected. The second main error contribution is caused bythe distribution of GPS time to the terminal. Both sources of errorsmanifest themselves in an uncertainty of the exact location of thecode/Doppler search window of the GPS receiver of the user equipmentthat is used to lock onto the ranging signal of one specific spacevehicle (SV). In WO06001738, methods and means for handling the searchwindow uncertainty caused by the initial position uncertainty aredisclosed. However, the determination and generation of the exactrelation between a satellite reference time, e.g. the GPS Time Of Week(TOW), and the timing of the cellular frame structure is not discussed.

The establishment of the time relation between GPS TOW and the timing ofthe cellular frame structure can be performed with two main methods. Oneway is to have dedicated reference GPS receivers in each radio basestation that time stamps the cellular frame boundaries of the uplink anddownlink connections. However, such solution calls for relativelyexpensive additional hardware in the radio base stations and havefurthermore redundancy drawbacks.

Another approach is to utilize measurements from user equipments ofopportunity, or from dedicated measurements, performed in A-GPS capableuser equipments. In the case of user equipment based A-GPS, these userequipments establish GPS TOW and can hence perform the time stamping,whereas a more complicated procedure has to be used for user equipmentassisted GPS. There, the sought relation is signaled to the positioningnode in the network.

The generation of basic measurements resulting in the fine timeassistance data is performed by the user equipment, which measures thenumber of chips to a cellular frame boundary, at a pre-determined GPStime. However, the establishment of GPS time in the user equipment isnon-trivial, in particular in user equipment assisted GPS, since therein prior art are ambiguous ways of interpreting the transmittedinformation.

If the GPS time in some user equipments is established correctly, theresults of the measurements are reported to the positioning node. In aWCDMA case, the positioning node is comprised in the RNC and the reporttakes place over the RRC protocol as a GPS Timing of Cell Framemeasurement. The positioning node collects the measurements andestablishes the relation between GPS TOW and the cellular frame timing,e.g. the UTRAN frame timing, for each cell of the cellular system. Thiscollection is not trivial either, also due to possibilities for theambiguous interpretation mentioned above.

When another user equipment is to be positioned making use of AGPS, thepositioning node prepares assistance data, and in particular fine timeassistance data. The data comprises an expected relation between thecellular frame timing, (e.g. UTRAN timing) and GPS time (TOW). This datais related to a specific pre-determined reference point, preferablylocated at the center of the cell. The data furthermore comprisesexpected code phase and Doppler shift at the reference point. Theuncertainty of the expected code phase and Doppler imposed by thespatial extension of the cell may also be provided, see e.g. WO06001738.This information is typically encoded by the positioning node e.g. as arecommended search window for each SV, expressed as a search windowcenter point and a search window length. However, the expected codephase and Doppler is also influenced by the mentioned error in therelation between the UTRAN frame timing and the GPS TOW. Without anyinformation about possible or probable uncertainties in that relation,the window for the expected code phase and Doppler to be searched has tobe made unnecessarily wide in order to cover all possible cases. This inturn leads to unnecessarily large computational efforts as well asunnecessarily long processing time.

SUMMARY

One problem with prior art satellite based positioning that relies onuser equipment based measurements for establishing relations betweencellular time and satellite time is that the accuracy of the establishedrelations are substantially unknown for the positioning node.Furthermore, standards of prior art cellular systems open up forambiguities upon reporting of time stamping of received satellitesignals from the user equipments, making the established time relationsbetween the cellular communication network and the satellite systemunreliable.

A general object of the present invention is therefore to provideimproved methods and arrangements for providing assistance data intendedfor positioning of user equipments in cellular communication systems,and thereby also to provide improved methods and arrangements forposition determination of user equipments in cellular communicationssystems. A further object of the present invention is to provide methodsand arrangements providing more reliable and efficient search windows.Another further object of the present invention is to provide methodsand arrangements ensuring non-ambiguous interpretations of time stampsof received satellite signals provided by user equipments.

The above objects are achieved by methods, arrangements, nodes andsystems according to the enclosed patent claims. In general words,reliable and efficient search windows are provided by allowing theadaptation of the search window to be additionally dependent oninaccuracy measures of relations between a cellular frame time and asatellite reference time. This inaccuracy is calculated in a positioningnode of the cellular communications system, preferably by filtering ofmeasurements received from user equipments. Linear trend Kalmanfiltering followed by postprocessing of estimation errors is presentlypreferred. In order to ensure non-ambiguous interpretation of thereceived time stamps of received satellite signals provided by userequipments, a pseudo propagation delay is computed in both the userequipment and the positioning network node. This computing is based onGPS acquisition assistance data available at both sides. The GPS timestamp is then defined referring to the determined pseudo propagationdelay, and may therefore be recreated at the network side in anon-ambiguous manner. In a preferred embodiment, the pseudo propagationdelay is assured to be situated within a pre-determined time interval.Such techniques are usable for several purposes, e.g. for refininginaccuracy determination of the relations mentioned above as well as forthe actual positioning of a user equipment.

One advantage with the present invention is that more efficient andreliable positioning of user equipments can be achieved, essentiallywithout needs for changes in standards of cellular communication systemreports. Another advantage with the present invention is that moreefficient and reliable positioning of user equipments can be achieved,essentially without needs for dedicated satellite reference nodes ineach base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating an example of A-GPS implementedin a cellular communication system;

FIG. 2 is a diagram illustrating relations between different systemtimes;

FIG. 3 is a schematic drawing of a cell in a cellular communicationsystem illustrating relations between position, synchronization andpropagation delays;

FIG. 4 is a diagram illustrating a relation between cellularcommunication system frame reference time and satellite reference time;

FIG. 5 is a block diagram illustrating an embodiment of a cellularcommunication system enabling A-GPS positioning of UEs according thepresent invention;

FIG. 6 is a flow diagram of main steps of an embodiment of a methodaccording to the present invention;

FIG. 7 is a diagram illustrating time relations at reception of asatellite ranging signal in a UE;

FIG. 8 is a diagram illustrating a preferred embodiment for computing apseudo propagation delay according to the present invention;

FIG. 9 is a flow diagram of main steps of an embodiment of anothermethod according to the present invention;

FIG. 10A is a block diagram of main parts of an embodiment of a cellularcommunication system arrangement according to the present invention; and

FIG. 10B is a block diagram of main parts of an embodiment of a userequipment according to the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of assisted GPS (A-GPS) implemented in acellular communication system 1. In this particular example, thecellular communication system 1 is a wideband code division multipleaccess (WCDMA) system. However, the principles of the present inventionare applicable also to other cellular communication systems as well. Inthis system a radio network controller (RNC) 20 acts as the node thatcollects refines and distributes assistance data to mobile terminals 10.The mobile terminals 10 are typically denoted as user equipment (UE) inWCDMA context, and the two terms will be used in the present disclosureas synonymous expressions. A core network (CN) 30 requests positioningof a UE over a Radio Access Network Application Part (RANAP) interface25. In response, the RNC 20 may use different kinds of A-GPS techniques.All these techniques do however build on assistance data being handledby a node, in the present disclosure denoted as positioning node, in thecellular communication system 1. The RNC 20 orders positioningmeasurements to be performed in the UE 10 via a base station 12 over theRadio Resource Control (RRC) interface 15. These measurements areperformed by dedicated A-GPS receiver hardware 11 in the mobileterminals 10. These GPS receivers detect GPS transmissions 55, typicallyGPS ranging signals, from the satellites 50. The satellites are oftendenoted as space vehicles (SV) 50.

In a GSM system, the positioning node is typically comprised in aserving mobile location centre (SMLC).

In order to facilitate the measurements of GPS ranging signals, the RNC20 provides the UE 10 with different types of assistance data. Theassistance data may e.g. be acquisition assistance data comprising codephase, integer code phase and the GPS bit number for a list of GPSsatellites as expected at a reference site. The assistance data may alsocomprise fine time assistance data, providing the GPS receiver 11 withhighly accurate information related to the global GPS time and satellitepositions in space. Such information can be used to calculate individualconditions for a particular GPS receiver 11. The GPS receiver 11 is insuch way typically provided with upper and lower bounds for code phasesof signals transmitted from the different satellites, when a search forthe satellite ranging signals 55 are to be performed.

The RNC 20 may obtain the assistance data in different ways. Oneapproach according to prior art, illustrated in FIG. 1 is that thecellular communication system 1 comprises a reference GPS receiver 40,which is able to detect the satellite ranging signals 55 and therebydirectly acquire fine time assistance data transmitted by the SVs 50 aswell as the GPS reference time. Assistance data and the GPS referencetime can then be provided to the base station 12 over a referencereceiver interface 45, allowing the creation of a relation in the basestation 12 between the cellular communication system clock and thesatellite system clock, i.e. time stamps of the UTRAN frame boundariesof the uplink and downlink connections expressed relative to the GPStime reference. Time stamps or filtered bias drift rates are typicallysent 39 over an interface 37 to the RNC 20. This solution is relativelysimple in its configuration. However, this solution requires theincorporation of a reference GPS receiver in connection to each RNC 20in the system, which renders in high additional costs. Furthermore,there are certain redundancy problems.

An alternative solution for obtaining at least the relation between thecellular communication system clock and the satellite system clock is touse UEs 10 of opportunity or from dedicated measurements in A-GPScapable UEs 10. This is not explicitly illustrated in FIG. 1, but the UE10 can be considered as one of the contributing UEs. In case of UE basedA-GPS (discussed more in detail below), the UEs 10 establish the GPS TOWfor a certain position of the cellular frame and can hence perform thetime stamping, whereas a more complicated procedure for UE assistedA-GPS is disclosed further below. The sought relation then has to besignalled to the positioning node in the network, in WCDMA preferablyover the RRC protocol 15.

At this point, it is suitable to mention that there are two types ofA-GPS positioning. One type is UE based A-GPS, which performs thepositioning calculation in the mobile terminal itself. The other type isUE assisted A-GPS, which performs only the ranging measurements in theterminal. The position is then calculated in a positioning node of thecellular communication system using the code phases measured in themobile terminal. For UE based A-GPS, the position and the GPS TOW isestablished in the UE.

In order to fully understand the problems and the solutions proposed bythe present invention, the relations between different system times haveto be discussed, preferably in connection to FIG. 2. A first GPS spacevehicle (SV) transmit a ranging signal 102 with a spectrum centered at1575.42 MHz. The signals include a so-called Coarse/Acquisition (C/A)code 108 that is unique for each SV. The C/A code 108 has a length of1023 chips and a chip duration of 1/1.023×10⁶ s. The C/A code 108repeats itself every 1 ms. Superimposed on the C/A code 108 is anavigation data bit stream with a bit period of 20 ms. The navigationdata includes among other things a set of so-called ephemeris parametersthat enables the receiver to calculate the precise position of thesatellites at the time of signal transmission. The SVs carry preciseatomic clocks to maintain clock stability.

The SV transmissions are however not perfectly synchronized to a generalGPS system time 101. The time axis is directed to the right in FIG. 2.By drawing a vertical line through the timing diagram of FIG. 2, one maytherefore obtain a snapshot of all clock readings as observed at variouslocations in space. The GPS system time 101 is defined as an ensembleaverage based on a set of ground station clocks and a subset of SVclocks. The individual SV clocks as experienced at the SV location, e.g.the clock of the first SV 102 may be slightly offset compared to the GPSsystem time, giving rise to a clock bias 112 for the first SV. AnotherSV 103 has another clock bias 113 etc. A model for the individualoffsets is transmitted as part of the navigation message from each SV.

A UE also comprises a clock 106A. This clock 106A is at least to acertain degree synchronized with the cellular communication systemclock, e.g. to the RNC frame structure, for enabling exchange ofmessages uplink and downlink. In case of fine time assistance, the UEclock 106A can also be expressed in terms of GPS system time frames106B. The clock 106B thus constitutes an estimate of the GPSTOW made inthe UE. The relation 107 between corresponding positions of the GPSsystem time 101 and the UE clock time 106B is the UE clock bias. Therelation between a frame time of the cellular communication system and asatellite reference time can e.g. be expressed as the GPSTOW for thestart of a particular reference RNC frame.

The clock bias 107 is typically relatively stable, but may exhibit minordrifts, which means that the positioning node of the cellularcommunication system continuously has to update the relation to the GPStime at least intermittently.

When the signals transmitted from the SVs reach a UE on the surface ofthe earth, they have been delayed with an amount depending on the rangefrom the SV in question to the UE. The delay depends on the distance thesignals have been traveling and is typically 58-87 ms for a UE locatedat or near the surface of the earth. The timing of the satellite signalsas received by the UE are illustrated in FIG. 2 by the lines 104 and 105for the first SV and the second SV, respectively.

A stand-alone GPS receiver normally needs to decode the completenavigation data stream before the receiver location can be calculated.This may take quite a long time, since e.g. the above mentionedephemeris and clock correction parameters are only sent once every 30 s.For some applications this delay may be unacceptably large, e.g. foremergency call applications. Furthermore, the decoding requires acertain minimum signal strength to be successful. The receiver candetermine the boundaries of the C/A code at much lower signal strengththan that required to decode the navigation messages. Thereforeso-called Assisted GPS methods were developed, wherein theaforementioned ephemeris and clock correction parameters are sent asassistance data on a faster and more reliable communication link, e.g. acellular communication link.

The assistance data typically also include an approximate GPS systemtime and the approximate location of the UE. Depending on the mode ofoperation, the UE may instead receive a set of parameters that enablesthe receiver to faster determine the C/A code boundaries. In the lattercase, the position is calculated outside the UE in a network positioningnode and the UE only provides the measurement of the relative positionof the C/A code boundaries.

The fundamental task of an A-GPS receiver is to measure the pseudorangeto a number of satellites. The pseudorange for a first satellite isdefined as:

ρ₁ =c·(t _(u) −t _(t1))  (1)

where t_(u) is the UE clock reading 106B at the time of reception, andt_(t1) is the time of signal transmission from the first SV 104 of thesignal portion received at the UE at time t_(u). The pseudorange differsfrom true range with a number of perturbing factors (receiver clockbias, ionospheric and tropospheric delays, SV clock bias, measurementerrors etc.) For the purpose of clarity, in this discussion, theinfluence of most of these error sources will be neglected. There areknown techniques to compensate for many of the above listed errorsources, see e.g. [1].

The simplified model is that the measured pseudorange obeys:

ρ₁ =|x _(u) −x _(s1)(t _(t1))|+b+e ₁.  (2)

Here x_(u)=(x_(u) y_(u) z_(u)) is a row vector containing thethree-dimensional coordinates of the unknown receiver location.Similarly x_(s1) is the row vector containing the coordinates of thefirst SV at time of transmission t_(ti). The SV moves at a speed of 3.84km/s so the transmission times need to be known at the millisecond levelunless the location accuracy will be degraded. The notation |z| meansthe norm of the vector quantity within brackets, which is equal to|z|=(zz^(T))^(1/2). In this case it can be interpreted as the distancebetween the receiver and the SV. Furthermore b is the receiver clockbias 107 (expressed as a range):

b=c·(t _(u) −t _(GPS))  (3)

where t_(GPS) stands for GPS system time. Finally e₁ is the measurementerror.

The fine time assistance data allows GPS receivers to obtain the bestsensitivity possible with A-GPS. To understand the benefits, it shouldbe mentioned that GPS is a code division multiple access (CDMA) system.The GPS signal from each satellite is hence associated with a specificcode. The chip rate of this code is 1.023 MHz for the civil coarseacquisition (C/A) signal. The signal from each satellite is retrieved bycorrelation against the unique code of each satellite. This code has aduration of 1023 chips, which adds up to exactly 1 millisecond. Afurther complication is that a 50 Hz bit PSK-modulated stream issuperimposed on the GPS ranging signal from the satellites comprisinge.g. the ephemeris data. Due to the PSK modulation, the bit edges maycomplicate ranging correlations since the unknown switches of sign atthe bit edges deteriorate correlation receiver performance in case theexact time instances of the bit edges are not known. Until accuratesynchronization to GPS time has been established in the GPS receiver,coherent correlation over more than 10 milliseconds is hence notpossible. This fact reduces performance significantly when the veryfirst satellite is acquired if no external synchronization informationis available. The acquisition of remaining satellites do not suffer fromthis sensitivity loss since they can exploit the exact synchronizationto GPS time obtained as a consequence of the detection of the firstsatellite.

To conclude, the first and most important benefit of fine timeassistance is that it allows the A-GPS receiver to apply coherentcorrelation detection also for the first satellite it acquires. The 5-10dB sensitivity gain that may be obtained is believed to be crucial toobtain consistent indoor coverage of A-GPS. The reason is that deepindoor, all satellites can be assumed to be equally weak. No singlesatellite is easier to detect than the remaining ones.

GPS correlation receivers search a two-dimensional code and Dopplerspace due to the large variation of the relative speeds of thesatellites. One advantage associated with fine time assistance is thatit allows the correlation search window to be reduced in the codedimension. This benefit is going to be explained further in connectionwith FIG. 3. A cell 2 is served by a radio base station (RBS) 12. Areference site 5, preferably in the centre of the cell 2 is defined.Fine time assistance data relating to the reference site 2 istransferred 13 to different UEs 10A-B within the cell. Satellite rangingsignals 55 are received at the UEs 10A-B. A specified time instant ofthe satellite signal 55 is to be detected by correlation to the C/Asignal. However, dependent on the actual position of the UE, thedetection time will differ. When a certain portion 124 of the satellitesignal 55 is received in e.g. a UE 10A situated at the reference site, aUE 10B situated closer to the boundary of the cell will have to wait atime 122 until the corresponding satellite signal portion 124 isreceived. Coherent correlation has to be performed over such largeportions of the satellite signal that these spatial differences will becovered. A search window has thus to be adapted accordingly.

However, if also the GPS synchronization has a certain uncertainty, alsothis will influence the size of the necessary search window. If an erroris present in the relation to the GPS system time data, the occurrenceof the portion 124 of the satellite signal 55 may appear as if it wasdetected at a later instant. This is illustrated as an offset of theframe boundary 126. The detection of UE 10B may then correspond to a UE10C having a correct synchronization but being positioned even outsidethe cell 2. An uncertainty in the synchronization between the UTRAN andthe GPS systems will thus add to the uncertainty introduced by thespatial uncertainty.

In order to adapt a search window to cover all alternativessynchronization has to be taken into account. A problem is then thatthere is presently no information entity in standard cellular controlsignaling, e.g. over the RRC uplink interface, that could provide thepositioning node by appropriate uncertainty measures of the measurementsperformed by the UEs. One approach could be to define a minimum requiredperformance for the determination of the synchronization, and compensatefor that. However, this will always give a search window that is alwaysadapted for the worst case of any UE that could be connected to thecellular communication network. This will not give any efficientadaptation of the search windows, which necessarily reduces theperformance. Another approach is to modify the standards, but such workmay take long time before agreed on. Furthermore, already existing UEshave to be adapted for such standard changes.

A reduction in the code dimension of more than a factor 10 as comparedto the complete 1023 chips code epoch of the GPS ranging signal can beachieved. This results in an additional A-GPS sensitivity improvementsince there are less code and Doppler search bins that can result infalse alarms of the receiver. This gain is however relatively small.Calculations indicate that it is of the order of 0.1-0.5 dB depending onthe assumptions. More importantly, the reduced search window sizesreduce the computational complexity of the GPS receiver proportionally,a fact that translates into the possibility to correlate for longerperiods of time to enhance sensitivity, or to reduce the computationtime, thereby also reducing the power consumption. The latter benefitmay be substantial in cases where the A-GPS receiver is used fortracking purposes during extended periods of time. Note that the benefitof a reduced search window is always present when new and undetectedsatellites are searched for.

According to what was discussed further above, a UE equipped with anA-GPS receiver can be utilized to determine the clock bias, or moregenerally a measure of a relation (or synchronization) between a frametime of the cellular communication system and a satellite referencetime. Such a measure can in turn be used for calculating by the cellularcommunication network to establish the time relation between thecellular and satellite frame structures. One aspect of the presentinvention relates to the establishment of such a time relation. At leasta part of the generation of fine time assistance data is performed bythe UE, which measures the number of UTRAN chips to an UTRAN frameboundary, at a pre-determined GPS time, known in the UE. Note thatestablishment of GPS time in the UE is non-trivial, in particular in theUE assisted case. The result of measurement is reported to thepositioning node, here the RNC, over the RRC protocol as a GPS timing ofcell frame measurement. This is discussed more in detail further below.According to the present invention, the positioning node collectssimilar measurements from UEs and establishes and tracks the relationbetween GPS TOW and the UTRAN frame timing, for each cell of thecellular system where A-GPS is supported.

In the next step, when a UE is to be positioned with A-GPS, thepositioning node prepares assistance data, and in particular fine timeassistance data. This data accounts for the expected (nominal) relationof the UTRAN frame timing in GPS time (TOW), defined at a referencesite, preferably in the center of the cell in which the UE is located.Furthermore, it comprises the expected code phase and Doppler at thereference site. Also, the uncertainty of the expected code phase andDoppler imposed by the spatial extension of the cell is provided,preferably according to WO06001738. However, also the uncertainty of theestablished relation between UTRAN frame timing and GPS TOW is ofbenefit.

One basic object of the present invention is to avoid the introductionof an information entity carrying measurement uncertainties informationabout the GPS timing of cell frame measurements of the UE, e.g. over theRRC to the positioning node. Another basic object of the presentinvention is to avoid the need to specify a minimum performance of theGPS timing of cell frames measurements of the UE. This is achieved bythe present invention by the application of a technique in thepositioning node that automatically estimates the uncertainty.

According to the present invention the time relation between the GPSsystem time and the cellular communication system time as well as theuncertainty of that relation are jointly estimated and tracked in thepositioning node, i.e. in the cellular communication network. Thisestimation and tracking is performed in the positioning node alone,leaving specifications and UE implementations unaffected.

This can be further illustrated by the diagram in FIG. 4. Relations 130between GPS time and UTRAN time are measured at a number of occasions,illustrated as points in the diagram. Each measurement 130 of therelation is associated with uncertainties 132, illustrated only for oneof the measurements. The value 130 is reported to the positioning node,however, the uncertainties 132 are unknown. According to the presentinvention, the relations are estimated, preferably by Kalman filteringusing a linear trend model. Estimation of the offset 136 as well as thegeneral slope 134 can thereby be achieved. The slope is theoreticallyequal to 1, but drifts in system clocks may introduce a minor deviationtherefrom. This can easily be handled by the Kalman filtering. Theoffset 136 corresponds to the time relation between the GPS system timeand the cellular communication system. The inaccuracy of this measure isalso easily obtainable. One way is to utilize a repeated differentiationof data. Another way is to apply a post processing step that is based onthe estimation errors obtained by the first estimation step. Standarddeviation is one possibility to express the uncertainty. Separateoutlier handling is also required, since there may be some UE modelsthat provide poor measurements or that suffers from the problemdescribed further below.

FIG. 5 illustrates a block diagram of a cellular communications systemhaving such estimation functionality according to the present inventionimplemented. A number of satellites 50 transmit ranging signals 55,detectable for a UE 10, The UE 10 measures the relation between the GPSsystem time and the cellular communication system and reports suchmeasurement over the RRC protocol 15 by a UTRAN signal 14 via the RBS 12to the RNC 20. The RNC comprises a positioning node 21. The positioningnode 21 predicts according to the present invention GPS timing of cellframes of a specific cell. A first estimator 22 in the positioning node21 receives measurement of GPS timing of cell frames and providesestimates of GPS timing of cell frames, e.g. using Kalman filteringbased on a drift model. The residuals of the estimation are provided toa second estimator 23 in the positioning node, where estimates of theuncertainty of GPS timing of cell frames are provided. These estimatedquantities are then used whenever a UE needs to be positioned.

A flow diagram of main steps of an embodiment of a method describing theprovision of assistance data according to the present invention isillustrated in FIG. 6. The process starts in step 200. In step 210satellite time reference data is provided. The satellite time referencedata comprises at least a relation between a frame time of the cellularcommunication network and a satellite reference time. This stepcomprises in turn the substeps 212, 214 and 216. In substep 212quantities representing the relation is measured in a multitude ofmobile terminals over time. The measured quantities are reported fromthe mobile terminals to the positioning node in step 214. In step 216calculations are performed. An estimate of the relation between theframe time of the cellular communication network and the satellitereference time is calculated from the reported measured quantities.Furthermore, an inaccuracy of the estimate of the relation is estimated.

In step 220, a search window for a specific satellite is adapted, basedin the satellite time reference data, and then in particular at least onthe estimated inaccuracy of the estimate of the relation between theframe time of the cellular communication network and the satellitereference time. Finally, in step 230, data representing the adaptedsearch window is provided to a mobile terminal that is to be positioned.The procedure ends in step 299.

In order to give some more details on a preferred embodiment of thefiltering process, the following linear trend model can be introduced asone example:

τ(t+T _(S))=τ(t)+T _(S)·{dot over (τ)}(t)+w _(τ)(t)  (4)

τ_(Measurement)(t)=τ(t)+e _(τ)(t)  (5)

Here τ is the GPS timing of cell frames value to be estimated, {dot over(τ)} the corresponding drift rate and w the model uncertainty, normallymodeled as Gaussian white noise. Furthermore τ_(Measurement) denotes themeasurements of GPS timing of frames and e denotes the measurement errorof one specific measurement. T_(S) denotes the measurement rate that canbe allowed to be time varying. A time variable Kalman filter can then bedesigned, see e.g. [2], for estimation of τ and {dot over (τ)}, usingthe measurements τ_(Measurement). It should be noted that there areseveral alternatives of varying level of sophistication as to the exactsolution of the filtering steps.

The estimation error from this filter can then be used as a basis forestimation of e.g. standard deviation that then forms the soughtuncertainty measure.

A less accurate alternative to the above procedure would be to simplynumerically differentiate the measurements twice. This would ideallyrender a measure of the measurement uncertainty of single measurements,that could then be combined in a step similar to the one outlined above.

An additional problem when utilizing UEs for providing measurements ofGPS-to-cellular frame relations is also that it is in present standardsnot perfectly determined how the measures reporting the GPS TOW shouldbe interpreted due to ambiguities introduced by truncation of data. Thiswill also be of importance during UE assisted A-GPS, where GPS timing isprovided from the UE to the positioning node.

FIG. 7 illustrates a time diagram of a detailed example of thedetermination of a transmission time determination for a receivedsatellite signal. A signal portion is transmitted at a transmissiontime, t_(t1), 111 from a satellite. The satellite signal portion 104 isreceived a specific time at the UE 110, t_(u). If t_(u) e.g. is definedas a frame boundary of the UE clock time, the corresponding transmissiontime, t_(t1) is to be determined by using the inherent timinginformation of the signal. This enables the determining a pseudorange ρ₁by multiplying a difference by the speed of light.

The transmission time t_(t1) is typically determined in several stages.First the submillisecond part of t_(t1) is determined by finding theboundaries of the C/A codes for the SV. This is done using correlatorsthat test all possible code phase and Doppler shifts within a certainsearch window, as described further above.

In a subsequent step, the millisecond part of the transmission timeneeds to be estimated. This only needs to be done for first SV to bedetermined. The complete transmission time of the subsequent SVs may bereconstructed in the RNC by using knowledge of one complete transmissiontime and apriori knowledge of UE and SV locations. Tentativetransmission times differ relative to each other by an integer number ofmilliseconds. One millisecond corresponds to 300 km. Therefore for mostcases, the transmission times would be possible to determineunambiguously if the complete GPS TOW information would be available forthe RNC. Thus, if GPS TOW is known, only the number of chips from thelatest frame boundary, δt_(ti), has to be determined, i.e. the truncatedtransmission time. From this a truncated pseudorange δρ_(i) can beprovided, using the relation:

δρ_(i) =c·(0.001−δt _(ti)).  (6)

The estimation of the millisecond part of transmission time requiresthat the received data is despread, leaving raw navigation data bits.The estimation can then be made by a number of techniques.

A first approach is direct demodulation of TOW. This requires first thatbit synchronization at 20 ms level is performed. Then the data isdemodulated at a rate of 20 ms. This process normally requires thatsubframe boundaries are determined followed by decoding the so-calledHandover Word, from which the TOW, i.e. the transmission time t_(t1) canbe derived. Each subframe has a length of 6 s, so this procedure mayrequire that approximately 8 seconds of navigation data is collected.TOW demodulation works down to approximately −172 dBW, assuming 0 dBantenna gain and is in fact the limiting factor for GPS coverage.

A second approach is TOW estimation using correlation techniques. Thisprocedure also requires that demodulation data bits are generated, butinstead of direct decoding, correlation is made with known transmittednavigation data bits. These bits comprises the so-called Telemetry Wordand the HOW word which may be sent to the UE as part of the assistancedata. This requires that the GPS time is apriori known within a fewseconds. This procedure works to somewhat lower signal levels thandirect TOW decoding, but most likely the performance is limited by thetracking loops that may loose lock at such low signal levels. Typicallyphase locked loops or automatic frequency control loops are employed forthis. However it is expected that this will work down to around −179dBW.

A third approach is the use of real time clocks. If TOW has previouslybeen determined, the receiver may be able to maintain an accurate clockat a millisecond level using e.g. the cellular system clocks thattypically drift only a few nanoseconds per second and long termstability may be better than 1 ms for a significant amount of time.However, it may be difficult for the user to know the absolute accuracy,which limits the use of this method. Furthermore, this method requiresthat either of the first or second approaches is performed with someinterval.

Whatever method is used, the UE supporting UE assisted A-GPS issubsequently required to compensate for the propagation delay and hencereport the approximate GPS system time at time of measurement, c.f. FIG.7. Note that the UE supporting UE assisted A-GPS normally does not haveaccess to ephemeris data, i.e. the satellite locations, nor does it haveknowledge of its approximate location. Therefore it appears to bedifficult to compensate for the propagation delay from the SV to the UE.Also, for position calculation it would not have been necessary toperform such compensation. A better alternative would have been toreport the estimated transmission time instead. In fact one of the firsttasks in the position calculation is to perform a transformation backfrom estimated GPS system time to the transmission times, since thetransmission times are needed for calculating the precise SV locationsat the time of transmission. The requirement to estimate GPS system timecreates a risk that the reconstruction of the transmission times cannotbe done correctly. This then introduces a millisecond ambiguity in therelation GPS/UTRAN time which makes the assistance data less useful oreven harmful.

According to one aspect of the of the invention, rules for how the UEshall perform the delay compensation are created, so that the RNCunambiguously can reconstruct the transmission times from the reportedGPS system time. Furthermore, the solution requires no change to thevarious cellular standards.

The UE is provided with the expected Code Phase (CP), Integer Code Phase(ICP) and the GPS Bit Number (GPSBN) for a list of SVs. These “codephase” parameters are hence valid at a specific time and at a particularreference location (c.f. FIG. 3). This common time stamp GPS TOW is alsoprovided. The data is provided in the information element GPSAcquisition Assistance, and the purpose is to enable the UE to reducethe number of code phase shifts in the search for the C/A code. For thisparticular application it is assumed that the UE does not receive anyaccurate relation between GPS and cellular time. Indeed, with such arelation available, the UE would not need to reconstruct the TOW atreception. It can therefore be assumed that the UE only knows GPS timewithin a few seconds. Therefore the above mentioned assistance datafields are useful only once the first SV signal has been acquired.

However, the information elements may surprisingly be used also for thepurpose of the present aspect of the present invention, i.e. toaccurately estimate the delay from the SV to the UE, typically for thefirst satellite. The precise definition of the mentioned assistance dataelements is as follows:

The Code Phase (CP) field contains code phase, in units of 1 GPS chip,in the range from 0 to 1022 GPS chips. The CP number defines the numberof chips that remain to the next C/A code boundary, as seen by areceiver at the reference site at the time of GPS TOW. This CP number isthus analogue to a truncated predicted pseudorange. The referencelocation would typically be an apriori estimate of the MS location. Thisfield is mandatory. The time resolution is obviously 0.001/1023 s.

The Integer Code Phase (ICP) field contains integer code phase, i.e. thenumber of the code periods that have elapsed since the latest GPS bitboundary, in units of C/A code period, as seen by a receiver at thereference site at the time of GPS TOW. This field is mandatory. Therange is 0-19 and the time resolution is 0.001 s.

The GPS Bit Number (GPSBN) field contains GPS bit number (expressedmodulo 4) currently being received at the time GPS TOW, as seen by areceiver at the reference site. This field is mandatory. The range is0-3 and the time resolution is 0.02 s.

It is obvious from these definitions that they are directly related tothe SV time as seen at a reference site at the time GPS TOW. By usingthe definitions, the transmission time from the first satellite can bederived as:

t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0

t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0,  (7)

expressed in seconds. The delay can thus be expressed as:

τ=t _(u,ref) −t _(t1,ref).  (8)

However, note that the transmission time is defined modulo 80 ms, whichmeans that the delay also need to be defined modulo 80 ms. Thet_(u,ref), i.e. the GPS TOW as expected at the reference site is alsoexpressed in units of 80 ms, which the results in:

τ=mod(t _(u,ref) −t _(t1,ref),0.08)=mod(−t _(t1,ref),0.08)=0.08−t_(t1,ref)  (9)

One here notices that the information contained by the GPS TOW,t_(u,ref), disappears, which renders the delay τ expressed in modulo 80ms an ambiguity. The delay r always gives a value between 0 and 80 ms.However, for the actual GPS satellites, the distances to the surface ofthe earth gives actual delays somewhere in the range of about 58-87 ms.

In FIG. 8, these relations are illustrated. At the horizontal axis isthe apparent delay, i.e. the delay modulo 80 ms given, and at thevertical axis is a pseudo delay illustrated. This pseudo delay differsfrom the actual delay due to certain errors and offsets, which can becompensated for in different ways. The pseudo delays are by thesatellite configuration known to be situated within a certain interval,δτ*. Apparent delays in the lowest region have to be interpreted asbeing reduced by 80 ms. Therefore, according to one preferred embodimentof the present invention, all apparent delays below a certain thresholdτ_(thr) should be compensated by adding 80 ms. This gives a relation 140between the apparent delay τ and the pseudo delay τ*. In order to coverthe intended pseudo delay range between 58 and 87 ms, the thresholdshould preferably be selected somewhere in the interval, δτ_(thr),ranging from 7 to 58 ms. To have some safety margins for errors andoffsets, and to also include mobile terminals not located exactly at thesurface of the earth, the threshold should be selected somewhere in themiddle of the interval, δτ_(thr), e.g. in the interval 30 to 45 ms.

If a specific τ_(thr) value is selected, the pseudo delay can becalculated as:

$\begin{matrix}\{ \begin{matrix}{{\tau^{*} = \tau},} & {{{if}\mspace{14mu} \tau} > \tau_{thr}} \\{{\tau^{*} = {\tau + {80\mspace{14mu} {ms}}}},} & {{{if}\mspace{14mu} \tau} \leq {\tau_{thr}.}}\end{matrix}  & (10)\end{matrix}$

The UE can now determine the transmission time t_(t1) from the firstsatellite as received at the actual location. The pseudo delay is thenused to determine the reception time, t_(tt), such that:

t _(tt) =t _(t1)+τ*.  (11)

The reception time is then quantized to integer milliseconds, which maybe done through a simple truncation, such that

GPSTOW=floor{1000*t_(u) }ms=floor{1000*(t _(t1)+τ*)}ms.  (12)

When the reports are sent to the RNC, also the transmission time t_(t1)is truncated, such that:

t _(t1) =k*10⁻³ +δt _(t1),  (13)

where δt_(t1) is the truncated transmission time, i.e. thesub-millisecond part, and k is an integer. This can also be expressed asa truncated pseudorange through the relation (6) further above. TheGPSTOW and the truncated pseudorange are reported to the RNC. In theRNC, the truncated transmission time is easily achieved, and the integerpart k may now be reconstructed by inserting (12) into (11) andrearranging it, giving:

k=GPSTOW−floor{1000*(δt _(t1)+τ*)}.  (14)

The value of τ* is not reported, but since the calculation of it isbased only on the assistance data provided from the RNC itself, acorresponding calculation of τ* can be performed also at the RNC side.The RNC can now use the reconstructed t_(t1) according to (12) alongwith the other pseudorange measurements and the RNC system frame numberto calculate the UE position, precise GPS system TOW and establish theprecise relation between GPS and UTRAN time.

It should be noted that other embodiments of the calculation of τ* canbe employed, as long as the same calculation is performed at both sidesin the cellular communication system, i.e. both at the UE and in theRNC. The calculation therefore has to rely on parameters that areavailable for both nodes. In the present invention, it was realized thatthe acquisition assistance data could serve this purpose.

FIG. 9 illustrates a flow diagram of main steps of an embodiment of amethod according to one aspect of the present invention. The procedurestarts in step 300. In step 310, GPS acquisition assistance data isprovided. This GPS acquisition assistance data comprises at least codephase, integer code phase and GPS bit number for a list of GPSsatellites as expected at a reference site. In step 312, the acquisitionassistance data is transferred to a UE. A plurality of satellite rangingsignals are received in the UE in step 314. In step 316, a pseudorangeto each of said plurality of GPS satellites and a GPS time stamp for thereception of respective signals are determined. This determinationincludes, for a first satellite to be determined, a computation of apseudo propagation delay based on the acquisition assistance data isperformed. The pseudo propagation delay is then used to determine theGPS time stamp for the reception of the first satellite ranging signal.In step 318, the time stamp and truncated pseudorange are transferred tothe positioning node. In case the time stamp is going to be used in afiltering procedure according to e.g. FIG. 6, this GPS time stamp andtruncations of pseudoranges are comprised in the reported measuredquantities representing the relation between a frame time of thecellular communication system and a satellite reference time. In step320, a calculation is performed. This calculation step comprises thesame computation of a pseudo propagation delay as was performed in theUE. This calculation may be e.g. a filtering procedure according to FIG.6. The calculation step may also comprise a calculation of a position ofthe UE based on the transferred GPS time stamp, truncations ofpseudoranges as well as on the computed pseudo propagation delay.

FIG. 10A illustrates a block diagram of main parts of an embodiment ofan arrangement 60 in a cellular communication system related topositioning of UEs. This arrangement 60 is typically comprised in apositioning node 21 of the cellular communication system, e.g. in a RNC20. However, the arrangement 60 may also be located elsewhere in thecellular communications system and also be configured in a distributedmanner, where different functionalities are comprised in different nodescommunicating with each other. The arrangement 60 comprises a sectionfor provision of data 61, in particular satellite time reference dataand/or GPC acquisition assistance data. The section for provision ofdata 61 is in the present embodiment connected to a section for adapting62 limits of a search window for assisting UEs in their search forsatellite ranging signals. The section for adapting 62 limits of asearch window and the section for provision of data 61 are connected toa transmitter 63, arranged for providing data, e.g. search window limitsor acquisition assistance data to a UE, preferably using differentcontrol signaling. The arrangement 60 further comprises a receiver 64,for reception of measures quantities representing the relation between aframe time of the cellular communication system and a satellitereference time and/or data representing truncations of a pseudorangebetween a plurality of satellites and the UE and a GPS time stamp of thereception of these ranging signals. The received data is provided to aprocessor 65. The processor 65 is in the present embodiment arranged forcalculating an estimate of the relation from the received data as wellas for calculating an estimated inaccuracy of the estimated relation.These estimates are provided to the section for adapting 62 limits of asearch window and the section for provision of data 61 in order toimprove the adaptation of the search window.

In the present embodiment, the processor further comprises means forcomputing 66 a pseudo propagation delay. This pseudo propagation delaycan be used in the calculation of the relation and/or for calculationsof a position of the UE.

FIG. 10B illustrated a block diagram of main parts of an embodiment of auser equipment 10 according to the present invention. The UE 10comprises a receiver 71 and transmitter 72 for control signaling withinthe cellular communication system. Through the receiver, acquisitionassistance data is received and provided to a processor 74. The UE 10additionally comprises a GPS receiver 73, arranged for allowingreception of satellite ranging signals from a multitude of GPSsatellites. The received ranging signals are provided to the processor74. The processor comprises means for computing 66 a pseudo propagationdelay, based on the received acquisition assistance data. The pseudopropagation delay is used by the processor to provide truncatedpseudoranges and corresponding GPS time stamps from the receivedsatellite ranging signals. Such truncated pseudoranges and correspondingGPS time stamps are provided to the transmitter 72 for further provisionto positioning nodes in the cellular communication system.

The embodiments described above are to be understood as a fewillustrative examples of the present invention. It will be understood bythose skilled in the art that various modifications, combinations andchanges may be made to the embodiments without departing from the scopeof the present invention. In particular, different part solutions in thedifferent embodiments can be combined in other configurations, wheretechnically possible. The scope of the present invention is, however,defined by the appended claims.

REFERENCES

-   [1] E. D. Kaplan, “Understanding GPS—Principles and Applications”,    Norwood, M A, Artech House, 1996, pp. 247-251, 255, 340-343.-   [2] T. Söderström, “Discrete-Time Stochastic Systems—Estimation and    Control”, Prentice Hall, Hemel Hempstead, UK, p. 235.

1. A method for providing assistance data when determining a position ofa mobile terminal being connected to a cellular communication system viaa base station, comprising the steps of: providing satellite timereference data; said satellite time reference data comprising a relationbetween a frame time of said cellular communication system and asatellite reference time; adapting a search window to a specificsatellite from which a satellite ranging signal emerges based at leaston said satellite time reference data; and providing said search windowto said mobile terminal; said step of providing satellite time referencedata in turn comprising the steps of: measuring quantities representingsaid relation in a multitude of mobile terminals over time; reportingsaid measured quantities representing said relation from said multitudeof mobile terminals to a positioning node of said cellular communicationnetwork; calculating, in said positioning node, an estimate of saidrelation from said reported measured quantities representing saidrelation, as well as calculating an estimated inaccuracy of saidestimate of said relation; whereby said step of adapting said searchwindow is based on said estimated relation and said estimated inaccuracyof said estimated relation.
 2. The method according to claim 1, whereinsaid step of calculating comprises Kalman filtering.
 3. The methodaccording to claim 2, wherein said Kalman filtering is based on a lineartrend model.
 4. The method according to claim 2, wherein said step ofcalculating further comprises postprocessing estimating said inaccuracyfrom estimation errors of said Kalman filtering.
 5. The method accordingto claim 1, wherein said step of calculating comprises estimation ofsaid inaccuracy from a repeated differentiation of said measuredquantities.
 6. The method according to claim 1, applied to mobileterminal assisted positioning using the global positioning system—GPS,comprising the further steps of: providing, in said positioning node,GPS acquisition assistance data comprising code phase, integer codephase and the GPS bit number for a list of GPS satellites expected at areference site; transferring said GPS assistance data to said mobileterminal; receiving, at said mobile terminal, signals from a pluralityof GPS satellites; determining, at said mobile terminal, a pseudorangeto each of said plurality of GPS satellites and a GPS time stamp for thereception of respective said signals; and transferring said GPS timestamp and truncations of said pseudoranges to said positioning node;whereby said reported measured quantities representing said relationscomprises said GPS time stamp and truncations of said pseudoranges; saidstep of determining a GPS time stamp and said step of calculating bothcomprising the further step of computing a pseudo propagation delaybased on said GPS acquisition assistance data; said step of determininga GPS time stamp for a first of said plurality of GPS satellites beingbased on said pseudorange to said first GPS satellite and said pseudopropagation delay; whereby a calculation of said relations being basedon said pseudo propagation delay and said reported measuredrepresentation of said relations.
 7. The method according to claim 6,wherein said computing of said pseudo propagation delay in turncomprises the steps of: determining an apparent propagation delay;establishing said pseudo propagation delay as said apparent propagationdelay if said apparent propagation delay is larger than a predeterminedminimum delay and as said apparent propagation delay plus 80 ms if saidapparent propagation delay is smaller than said predetermined minimumdelay.
 8. The method according to claim 7, wherein said minimum delay isselected in the interval of 7 to 58 ms.
 9. The method according to claim8, wherein said minimum delay is selected in the interval of 30 to 45ms.
 10. The method according to claim 7, wherein said apparentpropagation delay, τ, being calculated as:t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0 expressed in s, where GPSBN isthe GPS bit number, ICP is the integer code phase and CP is the codephase for said first of said plurality of satellites; said GPS timestamp for said first of said plurality of satellites being calculatedas:GPSTOW=floor{1000*(t _(t1)+τ*)}ms, expressed in ms, where t_(t1) is atransmission time from said first of said plurality of satellites and τ*is said pseudo propagation delay.
 11. The method according to claim 10,wherein said calculation of said relations is based on a transmissiontime calculated as:t _(t1)=[GPSTOW−floor{1000*(δt _(t1)+τ*)}]/1000+δt _(t1) expressed in s,where δt_(t1) is a truncated transmission time calculated according tothe relation:δρ₁ =c·(0.001−δt _(t1)) where δρ₁ is said truncated pseudorange.
 12. Amethod for determining a position of a mobile terminal in a cellularcommunication system, comprising the steps of: providing GPS assistancedata according to any of the claims 6 to 11; and calculating, in saidpositioning node, said position of said mobile terminal based on saidtransferred GPS time stamp, said transferred truncations of saidpseudoranges and said pseudo propagation delay.
 13. An assistance dataproviding arrangement, comprising: means for providing satellite timereference data; said satellite time reference data comprising a relationbetween a frame time of a cellular communication system and a satellitereference time; means for adapting limits of a search window to aspecific satellite from which a satellite ranging signal emerges basedat least on said satellite time reference data, connected to said meansfor providing satellite time reference data; and means for providingsaid limits of said search window to a mobile terminal; said means forproviding satellite time reference data in turn comprising: means forreceiving measured quantities representing said relation for a multitudeof mobile terminals over time; processor arranged for calculating anestimate of said relation from said received measured quantitiesrepresenting said relation, as well as for calculating an estimatedinaccuracy of said estimate of said relation; whereby said means foradapting said search window is arranged to adapt said limits of saidsearch window based on said estimated relation and said estimatedinaccuracy of said estimated relation.
 14. The arrangement according toclaim 13, wherein said processor is arranged for performing Kalmanfiltering.
 15. The arrangement according to claim 14, wherein saidKalman filtering is based on a linear trend model.
 16. The arrangementaccording to claim 14, wherein said processor is further arranged forpostprocessing estimating said inaccuracy from estimation errors of saidKalman filtering.
 17. The arrangement according to claim 13, whereinsaid processor is arranged for estimation of said inaccuracy from arepeated differentiation of said measured quantities.
 18. Thearrangement according to claim 13, for application to mobile terminalassisted positioning using the global positioning system—GPS, furthercomprising: means for providing GPS acquisition assistance datacomprising code phase, integer code phase and the GPS bit number for alist of GPS satellites expected at a reference site; means fortransferring said GPS assistance data to a mobile terminal; said meansfor receiving said measured quantities representing said relation beingarranged for receiving data representing truncations of a pseudorangebetween a plurality of GPS satellites and said mobile terminal, and aGPS time stamp for the reception of signals from respective ones of GPSsatellites in said mobile terminal; said processor being furtherarranged for computing a pseudo propagation delay based on saidassistance data; whereby said processor is arranged for calculating saidrelations based on said pseudo propagation delay, said pseudorange andsaid time stamp.
 19. The arrangement according to claim 18, wherein saidprocessor is arranged for performing said computing of said pseudopropagation delay by: determining an apparent propagation delay;establishing said pseudo propagation delay as said apparent propagationdelay if said apparent propagation delay is larger than a predeterminedminimum delay and as said apparent propagation delay plus 80 ms if saidapparent propagation delay is smaller than said predetermined minimumdelay.
 20. The arrangement according to claim 19, wherein said minimumdelay is selected in the interval of 7 to 58 ms.
 21. The arrangementaccording to claim 20, wherein said minimum delay is selected in theinterval of 30 to 45 ms.
 22. The arrangement according to claim 19,wherein said apparent propagation delay, τ, being calculated as:t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0 expressed in s, where GPSBN isthe GPS bit number, ICP is the integer code phase and CP is the codephase for said first of said plurality of satellites.
 23. Thearrangement according to claim 22, wherein said calculation of saidrelations is based on a transmission time calculated as:t _(t1)=[GPSTOW−floor{1000*(δt _(t1)+τ*)}]/1000+δt _(t1) expressed in s,where δt_(t1) is a truncated transmission time calculated according tothe relation:δρ₁ =c·(0.001−δt _(t1)) where δρ₁ is said truncated pseudorange,GPSTOW_(sv1) is said GPS time stamp and τ* is said pseudo propagationdelay.
 24. Cellular communication system node comprising an arrangementaccording to claim
 13. 25. The cellular communication system nodeaccording to claim 24, wherein said cellular communication system nodebeing a radio network controller—RNC—or a serving mobile locationcentre—SMLC.
 26. A method for determining a position of a mobileterminal in a cellular communication system, comprising the steps of:providing, in a positioning node of said cellular communication system,GPS acquisition assistance data comprising code phase, integer codephase and the GPS bit number for a list of GPS satellites expected at areference site; transferring said GPS acquisition assistance data tosaid mobile terminal; receiving, at said mobile terminal, signals from aplurality of GPS satellites; determining, at said mobile terminal, apseudorange to each of said plurality of GPS satellites and a GPS timestamp for the reception of respective said signals; transferring saidGPS time stamp and truncations of said pseudoranges to said positioningnode; and calculating, in said positioning node, said position of saidmobile terminal based on said transferred GPS time stamps and saidtransferred truncations of said pseudoranges; said step of determiningand said step of calculating both comprising the further step ofcomputing a pseudo propagation delay based on said acquisitionassistance data; said step of determining a GPS time stamp for a firstof said plurality of GPS satellites being based on said pseudorange tosaid first GPS satellite and said pseudo propagation delay; whereby saidstep of calculating being further based on said pseudo propagationdelay.
 27. The method according to claim 26, wherein said computing ofsaid pseudo propagation delay in turn comprises the steps of:determining an apparent propagation delay; establishing said pseudopropagation delay as said apparent propagation delay if said apparentpropagation delay is larger than a predetermined minimum delay and assaid apparent propagation delay plus 80 ms if said apparent propagationdelay is smaller than said predetermined minimum delay.
 28. The methodaccording to claim 27, wherein said minimum delay is selected in theinterval of 7 to 58 ms.
 29. The method according to claim 28, whereinsaid minimum delay is selected in the interval of 30 to 45 ms.
 30. Themethod according to claim 26, wherein: said apparent propagation delay,τ, being calculated as:t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0 expressed in s, where GPSBN isthe GPS bit number, ICP is the integer code phase and CP is the codephase for said first of said plurality of satellites; said GPS timestamp for said first of said plurality of satellites being calculatedas:GPSTOW=floor{1000*(t _(t1)+τ*)}ms, expressed in ms, where t_(t1) is atransmission time of said first of said plurality of satellites and τ*is said pseudo propagation delay.
 31. An arrangement for determining aposition of a mobile terminal in a cellular communication system,comprising: means for providing GPS acquisition assistance datacomprising code phase, integer code phase and the GPS bit number for alist of GPS satellites expected at a reference site; means fortransferring said GPS acquisition assistance data to said mobileterminal; receiver for receiving data representing truncations of apseudorange between a plurality of GPS satellites and said mobileterminal, and a GPS time stamp for the reception of signals fromrespective ones of GPS satellites in said mobile terminal; processorbeing arranged for computing a pseudo propagation delay based on saidGPS acquisition assistance data; said processor being further arrangedfor calculating said position of said mobile terminal based on saidtransferred GPS time stamps, said transferred truncations of saidpseudoranges and said pseudo propagation delay.
 32. The arrangementaccording to claim 31, wherein said processor is arranged for performingsaid computing of said pseudo propagation delay by: determining anapparent propagation delay; establishing said pseudo propagation delayas said apparent propagation delay if said apparent propagation delay islarger than a predetermined minimum delay and as said apparentpropagation delay plus 80 ms if said apparent propagation delay issmaller than said predetermined minimum delay.
 33. The arrangementaccording to claim 32, wherein said minimum delay is selected in theinterval of 7 to 58 ms.
 34. The arrangement according to claim 33,wherein said minimum delay is selected in the interval of 30 to 45 ms.35. The arrangement according to claim 32, wherein said apparentpropagation delay, τ, being calculated as:t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0 expressed in s, where GPSBN isthe GPS bit number, ICP is the integer code phase and CP is the codephase for said first of said plurality of satellites.
 36. Thearrangement according to claim 35, wherein said calculation of saidrelations is based on a transmission time calculated as:t _(t1)=[GPSTOW−floor{1000*(δt _(t1)+τ*)}]/1000+δt _(t1) expressed in s,where δt_(t1) is a truncated transmission time calculated according tothe relation:δρ₁ =c*(0.001−δt _(t1)) where δρ₁ is said truncated pseudorange,GPSTOW_(sv1) is said GPS time stamp and τ* is said pseudo propagationdelay.
 37. A cellular communication system node comprising anarrangement according to claim
 31. 38. The cellular communication systemnode according to claim 37, wherein said cellular communication systemnode being a radio network controller—RNC—or a serving mobile locationcentre—SMLC.
 39. A mobile terminal for mobile terminal assistedpositioning using the global positioning system—GPS, comprising:receiver for GPS acquisition assistance data comprising code phase,integer code phase and the GPS bit number for a list of GPS satellitesexpected at a reference site; processor for computing a pseudopropagation delay based on said acquisition assistance data; receiverfor signals from a plurality of GPS satellites; said processor beingfurther arranged for determining a pseudorange to each of said pluralityof GPS satellites; said processor being further arranged for determininga GPS time stamp for a first of said plurality of GPS satellites basedon said pseudorange to said first GPS satellite and said pseudopropagation delay; and means for transferring said GPS time stamp andtruncations of said pseudoranges to a positioning node in said cellularcommunications system.
 40. The mobile terminal according to claim 39,wherein said processor is arranged for performing said computing of saidpseudo propagation delay by: determining an apparent propagation delay;establishing said pseudo propagation delay as said apparent propagationdelay if said apparent propagation delay is larger than a predeterminedminimum delay and as said apparent propagation delay plus 80 ms if saidapparent propagation delay is smaller than said predetermined minimumdelay.
 41. The arrangement according to claim 40, wherein said minimumdelay is selected in the interval of 7 to 58 ms.
 42. The arrangementaccording to claim 41, wherein said minimum delay is selected in theinterval of 30 to 45 ms.
 43. The arrangement according to claim 40,wherein said apparent propagation delay, τ, being calculated as:t _(t1,ref)=GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10⁶, CP≠0t _(t1,ref)=GPSBN·0.02+ICP·0.001, CP=0 expressed in s, where GPSBN isthe GPS bit number, ICP is the integer code phase and CP is the codephase for said first of said plurality of satellites; said GPS timestamp for said first of said plurality of satellites being calculatedas:GPSTOW=floor{1000*(t _(t1)+τ*)}ms, expressed in ms, where t_(t1) is atransmission time of said first of said plurality of satellites and τ*is said pseudo propagation delay.
 44. Cellular communications system,comprising at least one arrangement according to claim
 31. 45.(canceled)